1. Field of the Invention
The present invention relates to thermal transfer printing, and more particularly, to a method and apparatus for printing bar code symbology, text or graphics onto a paper substrate in which an ink ribbon is advanced at a rate substantially lower than the advance rate of the paper substrate.
2. Description of Related Art
In the field of bar code symbology, vertical bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. The bar codes are typically printed onto paper substrate labels having an adhesive backing layer that enables the labels to be affixed to objects to be identified. To read the bar code, the bars and spaces are scanned by a light source, such as a laser. Since the bars and spaces have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the laser light that reflects from the bar code. In order to accurately read the bar code, it is thus essential that the bar code be printed in a high quality manner, without any streaking or blurring of the bar code. At the same time, it is essential that the adhesive backing layer of the labels not be damaged by heat generated during the printing process.
In view of these demanding printing requirements, bar code is often printed using thermal transfer printing techniques. In thermal transfer printing, an ink ribbon and a label sheet are pressed between a platen and a thermal print head. The thermal print head has linearly disposed printing elements that extend across a width of the label sheet. The printing elements are individually activated in accordance with instructions from a controller. As each printing element activates, ink from the ribbon at the location of the particular printing element is transferred onto the paper of the label sheet to produce the printed area. Both the ink ribbon and the label sheet are continuously drawn through the region between the platen and thermal print head, and in so doing, the bar code is printed onto the label as it passes through the region. Other images, such as text characters, are printed in the same manner.
The thermal transfer printer includes a mechanism for transporting both the label sheet and the ink ribbon from respective supply hubs to the print region. The transporting mechanism controls the feed rate of the label sheet and ink ribbon, and maintains a positive tension on the ink ribbon so as to prevent its wrinkling which could cause a defect in the printed bar code. If the ink ribbon were to stop temporarily under the thermal print head, the ribbon itself could be burned entirely through, causing the ribbon to tear. Traditionally, the ink ribbon was formulated so that all the ink is transferred from the ribbon during each successive pass across the label sheet. Thus, the ink ribbon must be transported at precisely the same rate as the label sheet to obtain substantially defect-free printing.
A significant drawback of this type of transporting mechanism is that it is wasteful of the ink ribbon. Since the printed area often represents only a limited portion of the overall label field, a quantity of the ribbon passes through the print region without transferring any ink to the label sheet. The unused ink can be salvaged by running the ink ribbon through the print region a second time with a new label sheet, though this method has an associated risk of defective printing if a previously transferred location of the ink ribbon reaches the print region at an instant in which a print operation is to occur.
A new formulation of ink ribbon allows only a portion of the ink to be transferred on each successive pass, permitting multiple transfers of ink from the same location on the ribbon. This capability enables the ink ribbon to be run through the print region two or more times before exhausting the ribbon, with less risk of defective printing. Nevertheless, each time the ribbon is passed through the print region, the transporting mechanism places stresses on the ribbon that stretch or weaken the ribbon, increasing the associated risk of tearing or wrinkling the ribbon. In addition, each pass of the ribbon results in lower print quality than that of the previous pass.
Accordingly, it would be desirable to provide a transporting mechanism for a thermal printer that is capable of taking advantage of the new formulation of ink ribbon by making most efficient use of the entire ink ribbon during a single pass through the print region. Ideally, the transporting mechanism would be capable of transporting the ribbon at a selectively slower rate than the label sheet, and would further be able to transport the ribbon at the slowest possible rate during the spaces between print operations.